310 DISTRIBUTION AND ZTIOLOGY OF THE CRAYFISHES. 
representation of the broad morphological differences 
which mark off the Potamobiide from the Parastacide 
Each group occupies a definite area of the earth’s surface, 
and the two are separated by an extensive border-land 
untenanted by crayfishes. 
A similar correspondence is exhibited, though less 
distinctly, when we consider the distribution of the 
genera and species of each group. Thus, among the 
Potamobiide, Astacus torrentium and nobilis belong essen- 
tially to the northern, western, and southern watersheds 
of the central European highlands, the streams of which 
flow respéctively into the Baltic and the North Seas, the 
Atlantic and the Mediterranean (fig. 77, I.); A. leptodac- 
tylus, pachypus, angulosus, and colchicus, appertain to the 
Pontocaspian watershed, the rivers of which drain into 
the Black Sea and the Caspian (I.); while Astacus 
dauricus and A. Schrenckii are restricted to the widely 
separated basin of the Amur, which sheds its waters 
into the Pacific (II.). The Astact of the rivers of 
western North America, which flow into the Pacific (IV.), 
and the Cambari of the Eastern or Atlantic water-shed (V.) 
are separated by the great physical barrier of the Rocky 
Mountain ranges. Finally, with regard to the Par- 
astacide, the widely separated geographical regions of 
New Zealand (VIII.), Australia (IX.), Madagascar (XII.), 
and South America (VI. and VII.), are inhabited by 
generically distinct groups. 
But when we look more closely into the matter, it will 
